gdb.info: Filenames

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List of filename extensions and languages

If a source file name ends in one of the following extensions, then GDB
infers that its language is the one indicated.
`.c'
     C source file

`.C'
`.cc'
`.cp'
`.cpp'
`.cxx'
`.c++'
C++ source file

`.m'
     Objective-C source file
`.f'
`.F'
     Fortran source file
`.mod'
     Modula-2 source file
`.s'
`.S'
     Assembler source file.  This actually behaves almost like C, but
     GDB does not skip over function prologues when stepping.
   In addition, you may set the language associated with a filename
extension.  *Note Displaying the language: Show.